The present invention relates to a process for the production of activated carbons by activating carbon-containing combustibles with phosphoric acid at elevated temperatures.
The chemical activation of carbon-containing combustibles, such as wood, straw, nutshells, bagasse, lignite, peat, etc. with phosphoric acid for the production of decolorizing carbons, is known.
In such an activation process, there is no direct action (as in the steam activation) in the carbon lattice, but primarily a dehydration process, as the end product of which the active carbon structure is secondarily formed.
It is known, in steam activation, to subject crude materials and/or crude products, to a pre-oxidation by treating with oxygen (for example, with air) at temperatures below the ignition temperature thereof.
The usual temperatures at which the pre-oxidation process (for example, for coal) is to be carried out in commercially practicable times, are from 200.degree. to 250.degree. C.
At these temperatures, however, a very substantial thermal decomposition begins in the combustiles, even before the oxidation process has been running to any noteworthy extent. By this decomposition of the material, the chemical and physical structure of the starting material is modified such that the subsequent chemical activation acts in a different manner and also more gradually than is the case with non-preoxidized material. Activated carbons from such a thermally pre-treated starting material, therefore, do not achieve the quality of a usual commercial product.
This is in accordance with the fact that temperature of about 100.degree. C. (that is of high temperatur drying) are sufficient to irreversibly destroy the partially colloid nature of these mentioned current combustibles and thus change the conditions for the decomposition by chemical activation. The decomposition of the unchanged cell structure of the starting material, however, substantially influences the quality of the end product. The advantage which a material, pre-oxidized in the usual manner, has to offer, has consequently so far not been used in a satisfactory manner in the case of easily decomposable crude materials, for the chemical activation based on cellulose, peat or lignite.
Experience has shown that chemical activates (i.e., activated carbons by chemical activation) which have been produced from non-pre-oxidized crude materials, can react more easily with atmospheric oxygen than those activates, whose production started from pre-oxidized crude materials. The increased reactivity in relation to oxygen results in more difficult handling in the case of chemical activates based on non-pre-oxidized crude materials.